1. Field of the Invention
This invention relates generally to electric vehicle battery pack thermal management and, more particularly, to a method and system for managing the temperature in an electric vehicle battery pack which allows the maximum temperature in the battery pack to rise as the state of charge of the battery pack drops, thus reducing energy expenditure on battery pack cooling and maximizing vehicle driving range, while still optimizing battery pack life.
2. Discussion of the Related Art
Electric vehicles are rapidly gaining popularity in today's automotive marketplace. Electric vehicles offer several desirable features, such as eliminating emissions and usage of petroleum-based fuels at the consumer level, and potentially lower operating costs. A key component of electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle's cost. Battery packs in these vehicles typically consist of numerous interconnected cells, operating at a relatively high voltage, and delivering a lot of power on demand. Maximizing battery pack life is a key consideration in the design and operation of electric vehicles.
In order to maximize battery pack life, the temperature in the battery pack must be controlled during charging and discharging so that it does not exceed a certain level. Control systems in existing electric vehicles typically prescribe a maximum allowable temperature in the battery pack irrespective of state of charge, and use an onboard thermal management system to prevent battery pack temperature from rising above the prescribed limit. However, using the thermal management system to extract heat during driving operation consumes energy, which detracts from the driving range of the vehicle. It has now been observed that the life of lithium-ion battery packs is not significantly degraded if the temperature in the battery pack is allowed to rise somewhat when the battery pack is in a reduced state of charge. The higher allowable temperature at lower battery pack states of charge allows reduced energy usage by the thermal management system.
There is an opportunity to employ a battery pack cooling methodology which ensures optimum battery pack life, but also maximizes vehicle driving range by not expending energy to remove heat from the battery pack if that heat is not detrimental to the battery pack. Such a method could yield cost savings for manufacturers of electric vehicles through the use of a smaller cooling system, and increase customer satisfaction through improved vehicle driving range.